So I have this huge national wetlands raster and I'm trying to set a minimum mapping unit of 5 acres. Meaning I want to delete clusters of pixels that are smaller than 5 acres. But the raster's attribute table only has one row - which means the entire raster is one massive clump of pixels. Is this possible? Or any suggestions on some other tool which might help me achieve that?

Cell size: 30x30
Number of bands: 1
Pixel type: signed integer
Pixel depth: 8 bit
Pyramids: level 9, resampling, nearest neighbor


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2 Answers 2


As whuber mentioned, the raster-based approach using Region Merge is blazingly fast compared to the vector based approach. These are the additional steps you will need to take to complete the analysis:

  1. Run Region Group (Spatial Analyst)
  2. Determine how many pixels make up 5ac in your raster dataset. If your cell size is 30m^2, then approximately 135 pixels encompass 5ac.
  3. Reclassify (Spatial Analyst) using the Count field in the Region Merge raster so that all groups of pixels < 135 = 0 and Count >= 135 = 1. Also keep in mind that you will want to reclassify the largest pixel count (i.e. non-wetland pixels) as 0.

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  • 2
    A much faster and expedient workflow stays in raster format and uses RegionGroup at the outset.
    – whuber
    May 21, 2014 at 19:32

Unlike vector data, pixels in raster datasets do not inherently belong to a given feature. The pixels simply have a value. So it's not so much that the entire raster dataset is one big clump, it's that all of pixels have the same value. You could assign each "clump" it's own value. While the resulting "zonal raster" could be used as an input for a number of functions (e.g., Zonal Geometry), I believe that accurately reclassifying the pixels in this way would be very difficult and time consuming.

I would consider Focal Statistics, which eliminates small variations in raster data, so that only the largest contiguous blocks remain. When using a majority statistic type, each pixel will be assigned the value that is most prevalent within a certain proximity of that pixel. So, if the user selects a 5x5 rectangular neighborhood, each pixel will receieve the value that is most prevalent in the 25 pixels closest to it. Because of this, small areas of isolated wetland surrounded by no wetland would be reclassified as "no wetland". While it may be difficult to eliminate clumps that fall below your 5 acre threshold exactly, this is a very effective tool to achieve what you're generally looking for. Try experimenting with different neighborhood sizes to see how the results vary.

You may also try converting the raster to polygon, calculting the size of the different polygons, and then doing a select by attribute to identify contiguous wetlands over 5 acres.

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